Mortensen is a painter, a published poet, a talented photographer and an actor of vast range, with roles encompassing an Amish farmer ("Witness," 1985), a king of Middle-Earth ("The Lord of the Rings" trilogy, 2001-2003), an assassin of mobsters ("A History of Violence," 2005) and a tattooed undercover Russian officer whose naked violent bathhouse brawl is one of the most memorable scenes in modern film history ("Eastern Promises," 2007).

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The latter role earned Mortensen an Academy Award nomination for best actor. Despite his successes, Mortensen believes that his most challenging performance has been portraying a starving father who struggles to protect his son in a post-apocalyptic America in his newest film, "The Road," based on Cormac McCarthy's Pulitzer Prize-winning novel.

"This movie was the culmination of the roles I've played. I put lots of pieces together from other things and mostly my life and poured it into this role. I feel like I somehow got a lot of things off my chest. It was cathartic," said Mortensen in an interview with ABC News Now's "Popcorn With Peter Travers."

Unlike many Hollywood blockbusters that equate the apocalypse with spectacular visuals of exploding world monuments, "The Road," directed by John Hillcoat, focuses instead on the human story. A father is trying to teach his son to be a "good guy," but "goes off the rails out of fear and paranoia to protect his son," Mortensen said. The father, while foraging for food and shelter, has to shield his son from bands of cannibals and thieving gangs.

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"Fear is used against us to make us do cruel things. Fear of what we don't understand and don't know. Fear defeats more people than anything else. How you deal with it makes you what you are," he added. In the end, it is the child who reminds his father of his teachings and becomes his "moral compass."

Mortensen believes they had to suffer to make a worthy adaptation of McCarthy's novel. "I knew it would be a tough journey," the actor said, "There are no shortcuts. You have to go there and take the audience there."

Making the movie was hard especially for then-11-year-old Kodi Smit-McPhee, who played Mortensen's son. Most of the filming was in Pittsburgh in winter, and Kodi, who hails from Southern Australia, had never seen snow. In a particularly intense scene, Kodi was so cold, he "started to shake," Mortensen recalled. "I knew he was falling apart emotionally. I could feel it. He's a mess. He's in pain. I was looking at him. I'm asking him do you want to stop? He stayed in character and from then on everything was different ... my admiration and my loyalty to him grew."

Mortensen was effusive in expressing his affection for his young co-star. "I really love this boy. We have an ongoing great relationship. That also was worth the experience," he said.

Kodi also brought much-needed levity to the set. "Kodi is such a prankster; he's a very good mimic. He can moondance as well as Michael Jackson could. He was a little variety show going on," recalled Mortensen.

The actor also had high praise for Charlize Theron, who plays his wife in the movie's flashbacks and added more depth to her role than originally given by the book. In McCarthy's novel, she chooses to end her life, and according to Mortensen, she is portrayed as either a coward or a bad mother.

In the movie, Theron transforms her role, and her point of view is portrayed as "more logical" than Mortensen's character. She asks her husband how they are going to survive and what is the point of life if merely for survival -- questions for which he has no answers.

For Mortensen and for his character, it is faith that keeps them going. They are optimists even in the face of defeat. "When we realize it's not going to get better, it's depressing, it takes the winds out of your sails, but then you look at what you already have and that's how you learn," he said. Mortensen's mantra is based on Ralph Waldo Emerson's quote, "Every wall is a door."

Mortensen is relying on his faith to promote his film, which practically has no marketing budget. He has taken it upon himself to get the word out by doing countless Q&As with audiences after screenings.

"Movies like this rely on word of mouth," he explained and claimed he has seen the movie's impact on its viewers. He wants people who have seen it to tell their friends and family, "I know you have heard the words bleak and depressing associated with it, but you're strangely stronger and more focused and more grateful to be alive at the end of watching this movie."

Mortensen's strongest critic will be his son, Henry, who at the time of the interview had not seen the film. His father depends on him for "tough and thoughtful analysis."

It was Henry, in fact, who made his father accept his most famous role -- Aragorn in Peter Jackson's "Lord of the Rings" trilogy and fans are clamoring for his return in "The Hobbit," which Jackson is co-writing and producing.

"I don't know, nobody's asked," responded Mortensen to the query of whether he would take part, but "I'd rather see myself than have another actor in my role."